System for low-cost connection of devices to an ATM network

ABSTRACT

A system for connecting low-cost, simple peripherals to an ATM network is disclosed. A plurality of such devices is connected to a standard ATM switch in a ring configuration. A polling station, located remotely in the ATM network, periodically generates a token cell which is delivered to the switch and, thereafter, is transmitted onto the ring configuration. The devices transmit cells in response to the token cells by transmitting cells behind the token cell as they forward it around the ring. The resultant cell chain is received at the ATM switch, where the cells are then sent onto the ATM network using virtual circuits.

TECHNICAL FIELD

The present invention relates generally to data communication networksand, more particularly, to the connection of low-cost, simpleperipherals to asynchronous transfer mode networks.

BACKGROUND OF THE INVENTION

Asynchronous Transfer Mode ("ATM") is an emerging packet switchingnetwork technology designed to provide service for a wide variety ofapplications such as voice, video and data. Originally proposed for usein the Broadband Integrated Services Digital Network ("B-ISDN") by theInternational Telegraph and Telephone Consultative Committee ("CCITT"),now reorganized as the Telecommunications Standardization Sector of theInternational Telecommunication Union ("ITU-T"), ATM is presently movingbeyond the wide area network setting into the private network arena as aplatform for local area networks ("LANs") with multimedia capabilities.ATM is now well known in the art and is described in various references.E.g., Martin de Prycker, Asynchronous Transfer Mode: Solution forBroadband ISDN (2nd Ed., Ellis Horwood Ltd., West Sussex, England,1993).

In an ATM network, as defined by the CCITT standards, information iscarried in packets of fixed size, specified for B-ISDN as 53 bytes oroctets, called cells. These cells are individually labelled byaddressing information contained in the first 5 bytes (octets) of eachcell. Although ATM evolved from Time Division Multiplexing concepts,cells from multiple sources are statistically multiplexed into a singletransmission facility. Cells are identified by the contents of theirheaders rather than by their time position in the multiplexed stream. Asingle ATM transmission facility may carry hundreds of thousands of ATMcells per second originating from a multiplicity of sources andtravelling to a multiplicity of destinations.

ATM is a connection-oriented technology. Rather than broadcasting cellsonto a shared wire or fiber for all network members to receive, aspecific routing path through the network, called a virtual circuit, isset up between two end nodes before any data is transmitted. Cellsidentified with a particular virtual circuit are delivered to only thosenodes on that virtual circuit.

The backbone of an ATM network consists of switching devices capable ofhandling the high-speed ATM cell streams. The switching components ofthese devices, commonly referred to as the switch fabric, perform theswitching function required to implement a virtual circuit by receivingATM cells from an input port, analyzing the information in the header ofthe incoming cells in real-time, and routing them to the appropriatedestination port.

The deterministic nature of ATM makes it ideally suited for carryingmultimedia or other real-time information, creating the opportunity tointerconnect a wide variety of devices or equipment which generate oremploy such information. For instance, an array of video surveillancecameras or motion sensors in an office building could be interconnectedvia an ATM network to efficiently deliver video images and other data toa common security control point. Further applications consist of thecontrol and monitoring of devices in a residential or commercialsetting. An individual may wish to program and/or monitor a microwave,refrigerator, clock, climate control, VCR, coffee maker, alarms, orother devices from one location in the home or at a remote location. Acomputer, either in the home or at a remote location, could useinformation from such devices to compile and transmit a shopping list,call a service specialist for the repair of a malfunctioning device,alert police to a security breach, or simply inform the homeowner ofanything unusual or interesting.

Unfortunately, the current interfaces required for connection to ATMnetworks make the use of these networks with such simple peripheralsprohibitively expensive. The major problem lies with current ATMprotocols which require that the switches and end stations support vastquantities of signalling and management software, including a base setconsisting of Q.2931, SSCOP, and SNMP. For a large class of possiblesimple ATM devices, such as those described above, the memory andprocessors required to implement these protocols is more expensive thanthe sum of the cost of the actual data handling components. Conceivably,lightweight versions of these protocols could be developed which wouldreduce the processing power necessary to implement an ATM interface fora simple device. However, even these protocols would require a minimumof software support which would be too costly for a large class ofapplications. Indeed, many devices could not be cost-effectivelyintegrated into an ATM network unless fully-hardware interfaces could beemployed. Furthermore, even if the necessity for this software supportcould be eliminated, the buffering in current ATM interfaces, in itself,could be prohibitively expensive.

Therefore, there exists a need for a mechanism to connect a number ofsimple peripherals to an ATM network using low-cost interfaces. Suchinterfaces would permit these devices to transmit and receive standardATM cells without significant buffering, and without need to supportcurrent ATM protocols.

SUMMARY OF THE INVENTION

The present invention is directed to a communication system forconnecting lowcost, simple peripherals to an ATM network. The systemcomprises a plurality of these simple peripheral devices interconnectedin series by a communication link. A switch connects the communicationlink to the ATM network, such that the communication link begins andterminates at the switch so as to form a ring. In this manner, cellstransmitted by any of the plurality of peripheral devices are deliveredto the switch and then further transmitted by the switch onto the ATMnetwork. Specifically, each peripheral device comprises an input portfor receiving data and an output port for transmitting data. The ATMswitch comprises a duplex switch port having an input connection forreceiving data and an output connection for transmitting data, where theoutput connection is connected to the input port of the first of theplurality of peripheral devices, each input port of the other peripheraldevices is connected to the output port of the previous peripheraldevice, and the output port of the last peripheral device is connectedto the input connection of the duplex switch port. Each of the pluralityof peripheral devices transmits data received at its input port on itsoutput port, so that data progresses around the communication link andis ultimately received at the switch.

In accordance with another aspect of the invention, a token cell istransmitted from the ATM switch onto the communication link, where it isreceived and forwarded by each of the plurality of peripheral devices,and ultimately returned to the switch. Each peripheral device transmitscells in response to the token cell. In one embodiment, each peripheraldevice may transmit a cell after transmitting the token cell and theexisting chain of cells previously transmitted by other devices. Inanother embodiment, each peripheral device may transmit a cellimmediately following the token cell, thereafter transmitting theexisting cell chain.

In accordance with another aspect of the invention, the token cell isgenerated by a polling station. The polling station may be located atthe ATM switch, but will most likely be a process running on a computerelsewhere in the ATM network connected to the ATM switch by one or morevirtual channel connections.

In accordance with another aspect of the invention, the peripheraldevices are addressed using address fields in the cell headers.Furthermore, these address fields are algebraically manipulated as theypass through each peripheral device, thereby permitting devices to beaddressed by their relative positions along the communication link.

In this manner, low-cost, simple peripheral devices can be connected toan ATM network without need to support current ATM protocols.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained byreading the following description in conjunction with the appendeddrawings in which like elements are labeled similar y and in which:

FIG. 1 is a diagram of an embodiment of a communication systemconstructed in accordance with the principles of the present invention;

FIG. 2A is a diagram of an ATM cell as defined by the CCITT;

FIG. 2B is a diagram of an ATM cell header at the User-Network Interfaceas defined by the CCITT;

FIG. 2C is a diagram of an ATM cell header at the Network-NetworkInterface as defined by the CCITT;

FIG. 3 (FIGS. 3A-3D) is an illustration of an example cell chaintransmitted on a first embodiment of the communication system of FIG. 1using an unslotted physical layer;

FIG. 4 (FIGS. 4A-4D) is an illustration of an example cell chaintransmitted on a second embodiment of the communication system of FIG. 1using an unslotted physical layer.

FIG. 5 (FIGS. 5A-5D) is an illustration of an example cell chaintransmitted on a third embodiment of the communication system of FIG. 1using a slotted physical layer; and

FIG. 6 (FIGS. 6A-6D) is an illustration of an example cell chaintransmitted on a fourth embodiment of the communication system of FIG. 1using a slotted physical layer.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be obvious, however, to one skilled in the art that the presentinvention may be practiced without these specific details. In otherinstances, well-known circuits, structures and techniques have not beenshown in detail in order not to unnecessarily obscure the presentinvention.

Furthermore, although what is described herein is a mechanism forconnecting lowcost, simple peripherals to an ATM network, it should beunderstood that the present invention is in no way limited inapplicability to ATM networks as defined by the CCITT. Rather, oneskilled in the art will recognize that the invention described hereinmay be employed in a wide variety of packet switching networks. Forexamples of some alternative networks, see de Prycker, pp. 50-58.

A basic block diagram of a communication system 100 constructed inaccordance with the principles of the present invention is shown inFIG. 1. Communication system 100 comprises a duplex ATM switch port 110connected to a plurality of simple peripherals or devices 120interconnected in a ring structure by communication link 130. Eachdevice 120 has an input port 122 and output port 124. Likewise, duplexATM switch port 110 has output connection 114 and input connection 112.Output connection 114 is connected via communication link 130 to inputport 122 of first device 120-1. Input ports 122 of each of the followingdevices 120 are connected via communication link 130 to output port 124of previous device 120, and output port 124 of last device 120 on thering, device 120-5 in FIG. 1, is connected to input connection 112 ofswitch port 110. Polling station 140 communicates with devices 120through switch port 110 via ATM network 150.

The wiring of communication link 130 may be easily implemented usingunshielded twisted pair cabling (UTP), such as the cabling employed inthe ATM25.6 standard. Those of ordinary skill in the networking fieldwill appreciate that a wide variety of cabling and physical media may beemployed to achieve the network topology depicted in FIG. 1. However, itwill be recognized that since one of the primary goals sought to beachieved by the present invention is the reduction of expense associatedwith networking arrays of simple devices, a low-cost physical mediumshould be chosen. The physical medium of the IBM 802.4 Token Ringstandard is well-suited for this purpose.

Although not necessary to the invention, standard ATM cells are employedthroughout the preferred embodiment of the present invention. Therefore,as an aid to understanding the invention, a description of a typical ATMcell is as follows. FIG. 2A shows the format of an ATM cell 200 asdefined in CCITT Recommendation I.361. This format, adopted for use inB-ISDN and other wide area networks, specifies a cell of 53 bytes(octets): an information field or payload 210 of 48 bytes (octets) whichcontains the user information which is the object of the transmissionand a cell header 220 of 5 bytes (octets).

Cell header 220, or simply "header", is used for transmitting a varietyof control information regarding the instant cell. FIG. 2B shows thestructure of this header at the User-Network Interface (UNI), that isthe interface between an end-user device and an ATM switch. Here theheader is made up of a Generic Flow Control (GFC) field 230 forspecifying information which may be used to control traffic flow at theuser-network interface, a VPI (virtual path identifier) 240, a VCI(virtual circuit identifier) 250, a payload type 260 which providesinformation regarding the type of information contained in the payloadarea 210 of the cell, a cell loss priority bit 270 for setting thepriorities relating to the abandonment of the cell during overloadconditions, and a Header Error Control field 280 which contains an errorcontrol check byte for the previous four bytes (octets) in header 220.

FIG. 2C shows the format of header 220 at the Network-to-NetworkInterface (NNI), the interface between network switches. This headerstructure is identical to the structure at the UNI except GFC 230 isreplaced with four additional bits of VPI 240. ATM networks do notprovide for flow control of the type which is implemented in some packetnetworks and ATM networks have no facility for storing cells over a longperiod of time. Therefore, inside an ATM network there is no need forgeneric flow control. Thus, within the NNI, GFC 230 may be eliminated infavor of an expanded VPI 240. For more information regarding standardATM cell formats see de Prycker, pp. 124-28. One skilled in the art willrecognize that the cell format described herein represents only onestandard cell format and that alternative cell formats and sizes may beemployed in conjunction with the present invention.

In order for switch port 110, and therefore polling station 140, to havefull connectivity with each device 120 attached to communication system100, devices 120 must pass on cells received on their respective inputports 122 to their output ports 124. As described in detail below, thepresent invention permits the devices to employ simple hardwareforwarding of cells, which eliminates the need for costly first-in,first-out (FIFO) memories. However, FIFO memories may be utilized inalternate embodiments, also described below.

It will be recognized that an arbitrary number of devices 120 may beinstalled in communication system 100, with the theoretical maximumnumber depending on the addressing range available, as each device 120must possess an unique address. If the eight bit VPI field 240 of a UNIformat cell header 220 is used as the vehicle for addressing, thistheoretical maximum will be 256. However, the accumulation ofregenerative clock jitter through the transceivers at input ports 122and output ports 124 of devices 120 creates a practical limitation onthe number of devices 120 which may be placed in communication system100. This phenomenon has been well-characterized with respect to the IBM802.4 Token Ring standard. Thus, if 802.4 transceivers are used, as theyare in the ATM25 standard, thirty-two (32) devices 120 is a sensiblemaximum design size. It will be recognized that those of ordinary skillin the art can readily calculate maximum desirable ring sizes foralternative physical layer implementations.

Communication along communication system 100 occurs in the followingmanner. A device 120 on communication system 100 will only transmit acell on its output port 124 after receiving on its input port 122 a"token cell." The token cell is a standard ATM format cell containingsome form of identifying information labelling it as a token cell. Forexample, the token cell may simply possess a VCI value outside the rangeused for addressing. Those of ordinary skill in the art will recognizethat the token cell may be identified in an almost limitless number ofways, so long as it possesses a unique pattern which cannot be confusedwith a normal cell containing user data.

Token cells are generated by polling station 140. Polling station 140may reside at ATM switch port 110, but in most implementations it willbe a process running on a nonlocal computer maintaining one or morevirtual channel connections with switch port 110 via ATM network 150.Polling station 140 periodically transmits a token cell onto ATM network150 where it is routed to ATM switch port 110. Switch port 110 transmitsthe token cell onto communication link 130 from output connection 114.Of course, if polling station 140 is resident at switch port 110, thetoken cell will be transmitted directly onto communication link 130without need to travel through ATM network 150.

All cells received by a device 120 on its input port 122 will beforwarded onto its output port 124. Therefore, after being transmittedonto communication link 130 by switch port 110 at output connection 114,the token cell makes a complete rotation through the loop of devices 120and is again received by switch port 110, this time at input connection112. At this point, the token cell may be discarded by switch port 110.Alternatively, the token cell may be returned to polling station 140where it can be detected as a loop integrity indicator demonstrating theproper connectivity of communication system 100.

A device 120 may only transmit a cell when it has received the tokencell on its input port 120. In one embodiment of the present invention,device 120 transmits by "tagging" a cell into the next free cell "slot"after the token cell. In this manner, after the token cell has passedthrough a series of transmitting devices 120, a chain of response cellswill be created following the token cell, which will be received byswitch port 110 at its input connection 112. It will be recognized thatthis embodiment is particularly suited for simple low-cost, hardwareimplementations of device 120, as no costly FIFO memories are required.Each device 120 simply retransmits the data it receives withoutbuffering until the end of the current cell chain, whereupon ittransmits its own cell.

Of course, physical layers, such as ATM25 or ATM100, which wouldtypically be employed in conjunction with the present invention arenon-slotted; therefore, special control characters are used to definethe necessary cell slots. After transmitting the token cell, switch port110 transmits a series of "idle" characters which represent the end ofthe cell chain. Each device 120 wishing to transmit searches for thefirst idle character after receipt of the token cell, whileretransmitting the data on its output port 124. This character willrepresent the end of the current cell chain and thus, the next free cellslot. Upon detecting this idle character, device 120 converts it to a"start of cell" (SOC) character and begins transmitting its cell ofdata. Device 120 will complete its transmission by again appending aseries of idle characters.

An SOC character is defined in the ATM25 and ATM100 standards. Likewise,an idle character is defined by the ATM100 standard and all data bytes54 or further beyond the last SOC in the ATM25 standard are implicitidles. These definitions may be used in conjunction with the presentinvention. However, it will be recognized by those of ordinary skill inthe art that any unique bit patterns which cannot occur within a properATM cell may be chosen as these characters.

It should be understood that device 120 must perform a "conversion" ofthe first idle character into a SOC character. Otherwise, its cell ofdata is likely to be overwritten by the next transmitting device 120.This conversion process requires that device 120 maintain at least aone-character buffer in its forwarding logic.

An example of this transmission method is depicted in FIG. 3, whichrepresents the circulation of one token cell where devices 120-1, 120-3and 120-4 have data to transmit. FIG. 3A shows the cell chainimmediately after transmission of the token by switch port 110 at outputconnection 114. The state of the cell chain is shown in FIG. 3B afterdevice 120-1 transmits its cell, in FIG. 3C after 120-3 transmits itscell, and in FIG. 3D after 120-4 transmits its cell. Note that an SOCcharacter must precede the token cell as well.

In a second preferred embodiment, each device 120 stores the token celland current appended cell chain into a FIFO buffer after receiving themat input port 122. After retransmitting the token cell at output port124, device 120 transmits its own cell of data followed by the existingcell chain partially stored in the FIFO. In other words, in thisembodiment communication system 100 acts as a register insertion ringwhere devices 120 always transmit their data cells immediately followingthe token cell. This approach is suited for use with currently existingnetwork interfaces. No modification of interface hardware is required.

An example of this second transmission method is depicted in FIG. 4,which again represents the circulation of one token cell where devices120-1, 120-3 and 120-4 have data to transmit. FIG. 4A shows the cellchain immediately after transmission of the token by switch port 110 atoutput connection 114. The state of the cell chain is shown in FIG. 4Bafter device 120-1 transmits its cell, in FIG. 4C after 120-3 transmitsits cell, and in FIG. 4D after 120-4 transmits it cell. As illustratedin FIGS. 4B-4D, intervening idle characters between cells may bepermitted in this embodiment without jeopardizing the integrity of thecell chain. The number of such idle characters should be kept small,however, as a string of idle characters can be used to detect the end ofthe cell chain.

The skilled artisan will recognize, however, that devices 120 employingeach of the above transmission methods may be installed in communicationsystem 100 simultaneously. Devices 120 employing the first method appendcells to the end of the cell chain, as described above, whereas devices120 utilizing the second method insert their cells immediately followingthe token cell.

Although the present invention would typically be used with unslottedphysical media, slotted media, such as the ATM155 physical layer, canalso be used. However, special control characters employed above tointroduce a dynamic slot structure to an unslotted layer are notnecessary when using slotted media.

FIG. 5 illustrates a third embodiment of the present invention whichemploys the transmission method of the above first embodiment on aslotted physical layer. Again, the illustration represents thecirculation of one token cell where devices 120-1, 120-3, and 120-4 havedata to transmit. FIG. 5A shows the cell chain immediately aftertransmission of the token cell by switch port 110 at output connection114. The state of the cell chain is shown in FIG. 5B after device 120-1transmits its cell, in FIG. 5C after 120-3 transmits its cell, and inFIG. 5D after 120-4 transmits its cell. As illustrated in FIG. 5,devices 120 with data to transmit simply transmit a cell in the firstempty cell slot following the token cell.

FIG. 6 illustrates a fourth embodiment of the present invention whichemploys the FIFO buffering transmission method of the above secondembodiment on a slotted physical layer. Again, the illustrationrepresents the circulation of one token cell where devices 120-1, 120-3and 120-4 have data to transmit. FIG. 6A shows the cell chainimmediately after transmission of the token cell by switch port 110 atoutput connection 114. The state of the cell chain is shown in FIG. 6Bafter device 120-1 transmits its cell, in FIG. 6C after 120-3 transmitsits cell, and FIG. 6D after 120-4 transmits its cell. As shown in FIG.6D, devices 120 in this embodiment have some lassitude to delaytransmitting, resulting in the interjection of an empty cell slotbetween data cells, without corrupting the cell chain.

Those of ordinary skill in the art will recognize that othertransmission scenarios are possible. For instance, devices 120 employingFIFO buffers could transmit their cells before forwarding the tokencell, rather than immediately following.

In all of the above embodiments, after the cell chain is received byswitch port 110 at input connection 112, switch port 110 will transmitthe cells to their intended destinations via ATM network 150. As thecells transmitted by devices 120 are standard ATM cells, they aretreated within ATM network 150 as any other cell travelling onconventional virtual circuits through intermediate ATM switches.However, often devices 120 will comprise some array of sensors or otherdata collection equipment whose data must be transmitted to the sameprocessor somewhere in the network, typically co-located with pollingstation 140. Therefore, bundling all of the cells onto a single virtualpath for this part of their journey would be more efficient. Such abundling can be achieved without effort where, as discussed below, asection of VCI 250 is used for addressing within the loop. If such isnot the case, switches which can encapsulate and de-encapsulate virtualpaths will be required. One such type of switch is the Virata™ ATMswitch from Advanced Telecommunications Modules, Ltd., Cambridge,England.

After completion of this transmission cycle, polling station 140 maytransmit another token cell to initiate another cycle. Those of ordinaryskill in the art will recognize that polling station 140 must delaysending a token cell such time as may be required to provide for allenvisaged transmissions in response to the last token cell. If the tokencell is normally returned to polling station 140 then the simplestmethod would be to wait for return of the token cell before sendinganother. However, this mechanism would not efficiently employ thebandwidth available to communication system 100, so a more frequenttransmission of token cells may sometimes be desirable. It should benoted that the source rate pacing function of most network interfacecards can be employed to achieve proper distribution of token cells.

A number of mechanisms exist for determining the optimum frequency oftransmission of token cells. For instance, polling station 140 may trackthe number of cells transmitted in response to token cells and increaseor decrease the frequency of the transmission of token cellsaccordingly. Therefore, if few devices 120 are transmitting cells, thefrequency of token cells may be increased by polling station 140.Alternatively, polling station 140 may assume a certain average ofresponse cells, and set the frequency of the transmission of token cellsaccordingly.

It will be recognized by those skilled in the art that requiring thereception of a token cell in order to transmit data yields a form of"backpressure" flow control, as is typically found in conventional ringnetworks. In other words, a device 120 may not transmit a cell until thedestination, here polling station 140, indicates that it is ready fordata by sending a token cell.

As is apparent from the examples illustrated in FIGS. 3-6, a device 120need not send data in response to the token cell. If a device 120 doesnot have data to send, it merely forwards the token cell on its outputport 124, along with any data following behind the token cell.

Furthermore, although the above illustrations show devices 120transmitting a single data cell in response to a token cell,communication system 100 could be implemented to permit devices 120 tosend multiple cells consecutively in response to the token cell. Thetotal number of cells a device 120 may transmit in response to a singletoken cell could be restricted by a system design policy or,alternatively, a dynamic algorithm could be employed which wouldguarantee that a device 120 does not consume an excessive amount ofsystem bandwidth. Even advanced scheduling algorithms such as the TimedToken Protocol of FDDI could be implemented, though such algorithms maybe prohibitively expensive for many typical implementations ofcommunication system 100.

Devices 120 receive cells by examining the addresses of cells which passthrough the ring and filtering those cells which they are interested in.Theoretically, cells intended for one or more devices 120 could betransmitted from any point in ATM network 150 to switch port 110, whichwould in turn place the cells onto communication link 130. However, in atypical implementation these cells will be transmitted by pollingstation 140 when needed, either periodically or occasionally. Theskilled artisan will recognize that polling station 140 can createbandwidth for these cells simply by holding off transmission of the nexttoken cell, although other known scheduling algorithms may be moresuited for some implementations.

Addressing of devices 120 is accomplished by placing device addresses ineither VPI 240 or VCI 250 in cell header 220. By using VPI 240 fordevice addressing, each device 120 can have access to its own virtualcircuit space, including its own set of reserved VCCs (in the range 1 to31). However, given the likely low-complexity of devices 120, manydevices 120 would only need one or a few VCCs to exchange single cellmessages using a custom ATM adaption layer (AAL). In this case, VCI 250or a portion thereof could equally be used for device addressing.Therefore, in the following description the term `address` refers tothose bits in cell header 220 that are significant in a particularimplementation.

Devices 120 possessing built-in identifiers may be addressed by simplyplacing the identifier in the address portion of cell header 220.Devices 120 without such built-in identifiers may be addressed using oneof a number of methods known to those skilled in the art. However,another method particularly useful in the present invention involvesalgebraic manipulation of cell headers 220. In this embodiment, alldevices 120 receive and transmit cells on a fixed hardwired address(such as 128). As cells are forwarded through a device 120, theforwarding logic manipulates the cell headers and adds one to theaddress. Using this implementation, it will be recognized that a device120 can be addressed explicitly by using as an address a number lessthan 128 by the position of device 120 along the chain. Receivedresponse cells are similarly distinguished by the increment theiraddress has encountered when received at polling station 140. It will berecognized that, if this method is used, HEC 280 must be recalculated ateach device 120. Of course, a device 120 may also respond to or transmiton a promiscuous address or set of addresses which are hardwired and nottranslated by the forwarding logic. One skilled in the art of networkswould appreciate that other implementations of algebraic manipulationwould be equally effective in identifying the correct origin anddestination of cells in a network of devices without built-inidentifiers.

The algebraic manipulation of cell headers 220 also provides a mechanismfor detecting the number of devices 120 installed in communicationsystem 100. For example, if a token cell is defined as any cell with anaddress in the range 192 to 255, and polling station 140 alwaystransmits the token cell with address 192 (or, alternatively, that isthe value after final header translation in ATM switch port 110), thenpolling station 140 may detect the loop length by examining the receivedtoken cell address after its passage through the loop.

In the absence of this capability, polling station 140 can crudelydetermine the length of the ring by measuring the time it takes fortoken cells to traverse the ring.

As should be apparent, the use of algebraic manipulation of cell headersto address a set of devices 120 connected in series is not restricted tothe network configuration shown in FIG. 1, but may be employed in otherconfigurations as well. Indeed, it can be implemented in any systemconnecting devices 120 in a daisy chain, including a chain which is anopen ended bus-structure, rather than a ring. For example, a number ofloudspeakers or other simple output devices could be interconnected inseries in an open-ended daisy chain. Sound samples or other data can betransmitted by an ATM switch port at the beginning of the chain, with anaddress offset determining which loudspeaker they will be played on.Sound destined for all loudspeakers can be carried on an address whichis received promiscuously by the devices and passed on withoutmodification.

It should be understood that various modifications will be readilyapparent to those skilled in the art without departing from the scopeand spirit of the invention. Accordingly, it is not intended that thescope of the claims appended hereto be limited to the description setforth herein, but rather that the claims be construed as encompassingall the features of patentable novelty that reside in the presentinvention, including all features that would be treated as equivalentsthereof by those skilled in the art to which this invention pertains.

What is claimed is:
 1. A communication system for connecting peripheralsto an ATM network, comprising:a plurality of peripheral devicesinterconnected in series by a communication link; and a switchconnecting the communication link to the ATM network, the communicationlink beginning and terminating at the switch so as to form a ring, suchthat cells transmitted by any of the plurality of peripheral devices arefurther transmitted by the switch onto the ATM network; each peripheraldevice comprising an input port for receiving data and an output portfor transmitting data; the switch comprising a duplex switch port havingan input connection for receiving data and an output connection fortransmitting data; the output connection being connected via thecommunication link to the input port of the first of the plurality ofperipheral devices; each input port of each of the other peripheraldevices being connected via the communication link to the output port ofthe previous peripheral device; the output port of the last peripheraldevice being connected via the communication link to the inputconnection of the duplex switch port; wherein each of the plurality ofperipheral devices transmits data received at its input port on itsoutput port, such that data progresses around the communication link tobe received at the switch; a token cell being transmitted by the switchonto the communication link and, thereafter, being forwarded in turn byeach of the plurality of peripheral devices back to the switch; eachperipheral device having data to send transmitting a cell on its outputport after receiving the token cell on its input port; and wherein atleast one peripheral device, after receiving the token cell on its inputport, transmits the token cell on its output port, transmits the cellstransmitted by previous peripheral devices, and then transmits its owncell if it has data to send, such that a cell chain is created which isreceived by the switch via the communication link; at least oneperipheral device having data to send transmitting its own cell afterdetecting an idle character following the token cell.
 2. Thecommunication system of claim 1 wherein the peripheral device convertsthe idle character to a start of cell character before retransmitting.3. The communication system of claim 2 wherein each peripheral devicetransmits a sequence of idle characters after transmitting its own cell.4. The communication system of claim 1 wherein at least one peripheraldevice having data to send transmits its own cell immediately followingthe token cell, followed by cells initially transmitted by previousdevices.
 5. The communication system of claim 4 wherein the peripheraldevice transmits a start of cell character before transmitting its owncell.
 6. The communication system of claim 5 wherein each peripheraldevice transmits a sequence of idle characters after transmitting itsown cell.
 7. The communication system of claim 1 further comprising apolling station for generating token cells.
 8. The communication systemof claim 7 wherein the polling station is located remotely from theswitch, the token cell being delivered to the switch via the ATMnetwork.
 9. The communication system of claim 8 wherein cellstransmitted by any of the plurality of peripheral devices are furthertransmitted by the switch to the polling station via the ATM network.10. The communication system of claim 9 wherein the token cell isreturned to the polling station via the ATM network as an indication ofthe integrity of the communication link.
 11. The communication system ofclaim 7 wherein the polling station adjusts the rate at which itgenerates token cells based upon the measured number of cells receivedin response to previous token cells.
 12. The communication system ofclaim 1 wherein the switch transmits cells intended for one or more ofthe plurality of peripheral devices onto the communication link.
 13. Thecommunication system of claim 12 wherein each cell includes an addressfield, each of the plurality of peripheral devices examining the addressfield of each cell it receives on its input port to determine whether itis an intended recipient of the cell.
 14. The communication system ofclaim 13 wherein the address field is algebraically manipulated by eachperipheral device before transmission on the output port of theperipheral device, such that each peripheral device may be addressedaccording to its relative position along the communication link.
 15. Thecommunication system of claim 14 wherein the algebraic manipulation isthe addition of one to the address filed.